Four water firms have been told to tackle high levels of complaints over service and billing problems.
Thames Water, Northumbrian Water, Essex & Suffolk Water and Hafren Dyfrdwy, which has seen complaints rise by 150%, must submit action plans and detail what is already being done.
Thames, England's largest supplier, had a 24% increase in written complaints over the past year.
Water UK, representing the companies, said numbers were now falling.
The Consumer Council for Water (CCWater) targeted the firms, stating customers are being forced to waste far too much time in dealing with the issues.
At Thames, 10% more customers were forced to phone to resolve an issue, the watchdog said.
The firm said the rollout of a new billing system was blamed for the increase in calls and written complaints to Northumbrian Water as well as Essex & Suffolk Water, which are both part of the Northumbrian Water Group.
Customers experienced longer call waiting times or saw them abandoned, leading to a 64% rise in written complaints to Northumbrian Water.
'Waste their time'
Hafren Dyfrdwy saw a 154% rise in written complaints during its transition from Dee Valley Water following a takeover by Severn Trent Water.
The firm blamed difficulties caused by a new bill layout and changes to charges.
CCWater chief executive Tony Smith said: "Far too many customers are having to waste their time and suffer the frustration of disputing unclear or inaccurate bills.
"In the autumn we'll be bringing the whole industry together to try and improve the standards of billing."
Anglian Water remains the best performer while South East Water, South Staffs Water, Hartlepool Water and Wessex Water also earned praise.
A Water UK spokesman said: "The latest data from CCWater, revealing there have been fewer calls made to water companies to resolve problems, shows that the industry's determination to provide the very best service for customers is working."
Southern Water has seen customer complaints reduced by 40% over two years.
The company said it had made "significant strides towards improving their standing in the industry".